The present invention relates to methods and apparatus for investigating the permeability of earth formations traversed by a borehole, and more particularly to novel and improved methods and apparatus for determining the permeabilities of formations by obtaining indications of characteristic response times of transient streaming potentials induced in the formation by the application of pressure pulses to the formation.
Heretofore, in some prior art practices, information relating to the location and permeability of subsurface earth formations has been obtained by electrical logging methods which are based at least in part on the electrokinetic potential phenomenon that occurs when relative movement is induced between a formation and the fluid contained in the matrices of the formation material. For example, in U.S. Pat. No. 2,814,017, issued Nov. 19, 1957 to Henri-Georges Doll, methods are described for investigating the permeabilities of earth formations by observing the differences in phase between periodic pressure waves passed through the formations and potentials generated by the oscillatory motion of the formation fluid caused by these pressure waves. Conversely, a periodically varying electric current was suggested to be used to generate oscillatory motion of the formation fluid, which in turn generated periodic pressure waves in the formation. Measurements were to be made of the phase displacement between the generating and the generated quantities and a direct indication of the relative permeability of the formation thereby obtained.
In U.S. Pat. No. 3,599,085, to A. Semmelink, entitled, "Apparatus For Well Logging By Measuring And Comparing Potentials Caused By Sonic Excitation", the application of low-frequency sonic energy to a formation surface is proposed so as to create large electrokinetic, or streaming, pulses in the immediate area of the sonic generator. In accordance with the disclosure of that patent, the electrokinetic pulses result from the squeezing (i.e. the competition of viscosity and inertia) of the formation, and the streaming potential pulses generate periodic movements of the formation fluid relative to the formation rock. The fluid movement produces detectable electrokinetic potentials of the same frequency as the applied sonic energy and having magnitudes at any given location directly proportional to the velocity of the fluid motion at that location and inversely proportional to the square of the distance from the locus of the streaming potential pulse. Since the fluid velocity was found to fall off from its initial value with increasing length of travel through the formation at a rate dependent in part upon the permeability of the formation rock, it was suggested that the magnitude of the electrokinetic potential at any given distance from the pulse provided a relative indication of formation permeability. By providing a ratio of the electrokinetic potential magnitudes (sinusoidal amplitudes) at spaced locations from the sonic generator, from which electrokinetic skin depth may be derived, actual permeability can in turn be determined.
Although these above-mentioned methods yield useful data relating to the borehole logging of subsurface formations, it is desirable to obtain permeability information through yet other methods which are believed to yield more useful results. More particularly, as provided by the present invention, pressure pulse excitation of the formation causes a transient flow to occur in the formation, and a measurement of the characteristic response time of the transient streaming potentials generated in the formation by such flow is employed to derive more accurate information relating to formation permeability.